Surface mount semiconductor diode devices typically comprise a semiconductor element electrically mounted on a first surface of a heat sink. A second opposing surface of the heat sink provides a first contact for the device. A conductive tab or leg is coupled to an opposing surface of the semiconductor element and is bent so that an end of the tab is substantially coplanar with the first contact, the end forming the second contact of the device.
In the surface mount diode U5ZA27 manufactured by Toshiba, the tab is connected directly to the surface of the semiconductor element. However, it is also known, for example with the surface mount diode MR2535S supplied by Motorola, Inc., to have an additional heat sink coupled to the opposing surface of the semiconductor element with the tab connected to the heat sink.
With both such types of devices, the surface areas of the first and second contacts are significantly large. Having large contact surface areas can lead to soldering difficulties when mounting such devices on, say, a circuit board. Ea addition, the heat sinks of such devices are large and the devices are arranged so that once mounted on a circuit board, the heat sinks are located on the board. With such arrangements, however, heat is transferred to the circuit board solder junction which can result in circuit board solder failures.
The semiconductor elements of both types of devices are encapsulated in a molding compound, typically molded epoxy, by injecting epoxy at high pressure. The stress caused by such a high pressure process step reduces the reliability of a device since molding stress can result in cracked die, mold voids and mold flashes.
The latter type of devices, in which the tab is connected to the heat sink, suffer from additional problems. Since the tab must be soldered to the heat sink, these type of devices require an additional step in their manufacture which increases their manufacturing cost. Furthermore, the step of attaching the tab to the heat sink typically requires a high temperature which may damage the semiconductor element leading to device failure. A further disadvantage with using tabs is that with the second process step of soldering the tab to the heat sink, it can be difficult ensuring that the tab is the correct height and hence that the first and second contacts are coplanar.
It would therefore be desirable to provide an improved surface mount semiconductor diode device in which the above problems and disadvantages are mitigated.